paintOrigin property

The visual location of the first visible part of this sliver relative to
its layout position.

For example, if the sliver wishes to paint visually before its layout
position, the paintOrigin is negative. The coordinate system this sliver
uses for painting is relative to this paintOrigin. In other words,
when RenderSliver.paint is called, the (0, 0) position of the Offset
given to it is at this paintOrigin.

The coordinate system used for the paintOrigin itself is relative
to the start of this sliver's layout position rather than relative to
its current position on the viewport. In other words, in a typical
scrolling scenario, paintOrigin remains constant at 0.0 rather than
tracking from 0.0 to SliverConstraints.viewportMainAxisExtent as the
sliver scrolls past the viewport.

This value does not affect the layout of subsequent slivers. The next
sliver is still placed at layoutExtent after this sliver's layout
position. This value does affect where the paintExtent extent is
measured from when computing the SliverConstraints.overlap for the next
sliver.

Defaults to 0.0, which means slivers start painting at their layout
position by default.